Because of the damaging effects on contrast rendition by obstructions in the light path of a telescope, several attempts have been made to design easy-to-make and collimate unobstructed telescopes comprising tilted rotationally symmetric optical elements.
Telescopes with tilted components have been described in the following publications:
1. A. Kutter, Der Schiefspiegler, 1953 (Abstract: The Schiefspiegler, Bull. A, Sky Publ. Corp., Cambridge, Mass.) PA0 2. R. A. Buchroeder, Technical Report No. 68, Opt. Sc. Center, Univ. of Ariz., 1971 PA0 3. R. A. Buchroeder and A. S. Leonard, A.O., Vol. 11, No. 7, 1972, p. 1649 PA0 4. A. S. Leonard, in Adv. Tel. Making. Techniques, Vol. 1, Willmann-Bell, 1986, p. 231 PA0 4. W. B. King, A.O., Vol. 13, 1974, p. 21 PA0 5. R. Gelles, O.E., Vol. 13, 1974, p. 534 PA0 1. U.S. Pat. No. 4,226,501 PA0 2. U.S. Pat. No. 4,265,510 PA0 1. Residual aberrations, i.e. only systems with moderate entrance pupil diameters could be realized. PA0 2. An unfavourable position of the focal surface. PA0 3. A reversed image because of an odd number of reflections. PA0 4. Toroidal surface figures are needed. PA0 5. A tilted image plane. PA0 6. Anamorphic distortion PA0 7. Surfaces with extremely long focal length. PA0 1. An unobstructed light path in order to achieve a maximum of contrast rendition and resolving power. PA0 2. The focal plane position should be readily accessible and as close to the telescopes center of gravity as possible. PA0 3. A slow focal ratio (.apprxeq.F/15 to .apprxeq.F/20) in order to achieve high magnifications with long focus eyepieces and long eye relief. This also suppresses aberrations that may, otherwise, be introduced by prism type binocular viewers. PA0 4. A relatively short (secondary) tube, less than 40% of the effective focal length. PA0 5. No extreme long radii of curvature of any of the mirrors involved. PA0 6. The surfaces should be easy to manufacture, i.e. spherical wherever possible. PA0 7. Tolerances should be loose and collimation easy without introducing optical disadvantages. Thus, telescopes of the tilted component class had to be chosen. PA0 8. Aberrations should be better corrected as in the prior art systems to make bigger telescopes possible. PA0 9. Light should be reflected four times to obtain an astronomical image orientation. PA0 10. It should be possible to minimize or even eliminate image plane tilt and/or PA0 11. distortion. PA0 12. A convex surface should have a concave counterpart to avoid manufacture of an additional master surface for testing. PA0 13. Complete blocking of stray light should be possible without vignetting or extending the tube. PA0 14. The systems should be completely free from chromatic aberrations, i.e. reflecting surfaces had to be used throughout to make the instruments in any spectral range usable. PA0 15. The systems should be equipped with concave primary and convex secondary mirrors because coma astigmatism and spherical aberration tend to cancel each other, i.e. Schiefspiegler type systems.
and systems with tilted components for laser-beam transmission in:
In addition, more general prior art other than tilted component telescopes is given, for example, in the following publications:
The inherent advantage of the tilt-and-decenter philosophy is simplicity itself. Each optical element is rotationally-symmetric and is being treated during the design and fabrication as an independent unit. The origins of local coordinate systems and the vertices of the surfaces coincide. Tilted-component systems do not have an optical axis in the common sense. Instead, it is defined by a ray joining an object point and subsequently the vertices of the optical elements and an image point. The tilt aberrations are cancelled in such a way that the individual elements of the optical systems are arranged to make the sum of the aberrations zero or at least as small as necessary, instead of deducing solutions from already known systems or using a predetermined geometry.
It follows from this kind of aberration correction that no predetermined geometry like certain radii of curvature, air spacings or tilt angles exists. Most of the surfaces to be made are spherical and their radii of curvature must not be necessarily closely kept. Resulting additional aberrations can be cancelled during collimation by changing distances and/or tilts without any disadvantage.
Until now there have been introduced in the tilted-component class: the Schiefspieglers with concave primary and convex secondary by A. Kutter; the Yolo-systems as well as the Solano reflector with concave primarys and concave secon-darys by A. S. Leonard (R. A. Buchroeder, "Design Examples of TCT's", OSC Rep. #68, 1971); furthermore, three-mirror Schiefspieglers by R. A. Buchroeder and A. Kutter and several catadioptric systems (ibidem).
Despite these efforts, a closer examination of the proposed optical systems indicates that each of them shows one or more of the following disadvantages:
To overcome the third of the above mentioned drawbacks some users of the three-mirror Schiefspieglers added a plane mirror. However, a fourth mirror offers further degrees of freedom to improve on the known systems.